UnitRegistry.convert now returns a Fraction if the user submits a Fraction to it. I also added a test for this issue.

pint/testsuite/test_issues.py

@@ -507,3 +507,9 @@ class TestIssuesNP(QuantityTestCase):
         t = copy.deepcopy(q)
         u = t.to(ur.mF)
         self.assertQuantityEqual(q.to(ur.mF), u)
+
+    def test_issue323(self):
+        from fractions import Fraction as F
+        self.assertEqual((self.Q_(F(2,3), 's')).to('ms'), self.Q_(F(2000,3), 'ms'))
+        self.assertEqual((self.Q_(F(2,3), 'm')).to('km'), self.Q_(F(1,1500), 'km'))
+

pint/unit.py

@@ -18,6 +18,7 @@ import functools
 import operator
 import pkg_resources
 from decimal import Decimal
+from fractions import Fraction
 from contextlib import contextmanager, closing
 from io import open, StringIO
 from collections import defaultdict
@@ -1109,6 +1110,9 @@ class UnitRegistry(object):
         if isinstance(value, Decimal):
             factor = Decimal(str(factor))
 
+        if isinstance(value, Fraction):
+            factor = Fraction(Decimal(str(factor)))
+
         if inplace:
             value *= factor
         else: